Several different methods for intra prediction for Reduced Resolution Update (RRU) based video encoders and decoders (CODECS) have been proposed. In a first prior art method for intra prediction for RRU based CODECs, the spatial intra prediction methods in H.264 using either 4×4 or 16×16 block size are extended to 8×8 and 32×32 block sizes for RRU mode. The residue is downsampled and is coded using the same transform and quantization process in H.264. During decoding, the residual data needs to be upsampled. The downsampling process is done only in the encoder, and hence does not need to be standardized. However, the upsampling process must be matched in the encoder and decoder, and so must be standardized.
In a second prior art method for RRU based video CODECS, an intra prediction method is proposed to improve the subjective quality of the video. First, the original full resolution samples are downsampled, and then a low resolution prediction is created based on these new samples. This low resolution prediction is upsampled to full resolution (i.e., using zero order hold) and used for prediction. It is also proposed in this method/apparatus to consider odd samples for odd positions, and even samples for even positions. This method improves the subjective quality but still has many artifacts on edges.
In a third prior art method for intra prediction in RRU based CODECS, it is proposed to generate prediction blocks for Intra coding. Rather than work on the full resolution image, the pixels neighboring the 32×32 coding region are downsampled and placed in a temporary buffer. Next, the intra prediction is done the same as is done in the JVT standard implementation, with prediction decisions made on each 4×4 block. After the prediction and residual coding is complete, the intra macroblock is upsampled to replace the macroblock in the reference image. The shortcoming of this approach is that all high-frequency prediction information is lost during the downsampling process. This method also does not work in conjunction with an RRU based complexity scalable video CODEC. Since this approach requires downsampling of the reference pixels for intra prediction, the upsampling filter as well as the downsampling filter must be standardized.
In the third prior art method, the encoder downsamples the neighboring pixels of the 32×32 macroblock to perform the intra prediction on a 16×16 macroblock and after the prediction and residual coding is complete, the intra macroblock is upsampled to replace the macroblock in the reference image. When the pixels of this intra macroblock are used for intra prediction of its neighboring macroblock the same procedure of downsampling and upsampling is carried out. Since the upsampling and downsampling are non-reversible processes this results in a loss of details. This method also does not work for the RRU based complexity scalable video CODECs. Particularly, for the low resolution decoding, this downsampling and upsampling of the neighboring pixels results in a mismatch at the decoder, the visual impact of which is extremely severe.
Therefore, current intra prediction methods for RRU video CODECS do not perform well on block edges. They result in very prominent jagged edges. This has a severe impact on the visual quality of the video sequence.
Accordingly, it would be desirable and highly advantageous to have a method and apparatus for performing intra prediction for Reduced Resolution Update (RRU) mode that overcomes the above-described problems of the prior art.